This invention pertains to electronic test instruments and more specifically to a test instrument suitable for measuring performance of microwave radio receivers.
Radio systems are becoming considerably more complex in an attempt to utilize radio spectrum more efficiently. For example, multi-level modulation techniques such as 64Q AM utilize quadrature modulation (i.e. modulation of both the carrier frequency and phase) to provide a number of discrete states representing binary data. 64QAM provides 64 such discrete modulation states;256QAM provides 256 discrete modulation states. While more sophisticated modulation techniques allow more efficient use of the radio spectrum, sophisticated modulation techniques are, unfortunately, more sensitive to the effects of noise and other propagation errors.
The error performance standards imposed on radio equipment is specified by the CCITT and CCIR. Such performance standards define reception "outage" limits, i.e. error limits beyond which a receiver is considered to be out of service, i.e. not properly receiving the incoming signal.
W. D. Rummler has characterized multi-path fade mathematically as: EQU H(jw)=a[1-B e.sup.-j(w-w.sbsp.0)t]; where (1)
B=-20 log(1-b), the notch depth in dB; PA0 A=-20 log (a), the fade in dB; PA0 A+B=the total fade at the minimum response; PA0 t=a delay term (t&gt;0 for minimum phase, t&lt;0 for non-minimum phase with minimum and non-minimum phase as defined in network theory);. PA0 w=is the frequency; and PA0 w.sub.0 =the notch center frequency
FIG. 1 is a plot of the signature or "M curve" of a typical microwave receiver. The horizontal axis depicts frequency, with a center frequency at f.sub.0. The vertical axis depicts notch depth, in decibels, required to produce a specified error ratio, for example 10.sup.-3.
Points located within the signature (i.e. inside the M curve) correspond to a receiver data error ratio greater than the specified error limit, and therefore represent the outage region of the receiver. The use of signature plots or M curves, as shown in FIG. 1, allow microwave receivers to be characterized, and thus compared for relative merit.
It is well known that multi-path fading, a major contributor to microwave radio receiver error rates, is dynamic in nature and thus a proper signal analysis should simulate dynamic multi-path fading in order to properly characterize the microwave radio equipment.
One prior art technique for simulating multi-path fading is shown in the block diagram of FIG. 2. A microwave radio signal is applied to input terminal 21, and then split into two paths utilizing splitter 22. Path 23a includes time delay element 24, phase shifter 25, and attenuator 26. Path 23b includes only attenuator 27. The output signals from paths 23a and 23b are combined in combiner 28, providing a microwave output signal on output terminal 29 which includes multi-path distortion since the input microwave signal has traveled over two separate paths just as a microwave radio in the atmosphere may travel over two paths, e.g. a direct antenna-to-antenna link and a link between antenna-to-antenna which reflects off of the earth's surface.
One disadvantage of the structure of FIG. 2 is that it operates at microwave frequencies, and thus is expensive, cumbersome, and difficult to accurately adjust. It is particularly difficult to accurately generate notches at microwave frequencies of desired frequency and depth. This, of course, adversely affects the the accuracy of the simulation.
Another prior art technique for performing signature analysis of microwave radios uses the intermediate frequency (IF) signal corresponding to a received microwave signal undergoing multi-path distortion. This IF signal from the microwave radio under test is given simulated multi-path distortion and then injected into the IF signal path of a microwave receiver, and the error analysis performed in order to obtain the signature curve. This technique obviates the problems of generating a multi-path distortion simulation signal at microwave frequencies. It is, of course, considerably easier to provide notches of desired and accurate frequencies and depths at an intermediate frequency of, for example, 70 or 140 MHz, as compared with providing notches of accurate frequency and depth at microwave frequencies.
One example of such an intermediate frequency multi-path distortion simulator for the purpose of this signature measurement of microwave equipment is described by Richman, "Automated Signature Measurement of Operational Microwave Radio-Relay Equipment Using a Novel Multi-Path Simulator", IEEE International Conference on Measurements for Telecommunication Transmission Systems-MTTS 85, Conference Publication No 266, pages 108-111. Such IF simulators can take a form of the microwave simulator shown in FIG. 2, with of course RF components operating at the IF frequency substituted for the microwave components described with regard to FIG. 2.
Alternatively, a structure as shown in FIG. 3 is used in the prior art in which path 33a includes phase shifter 35 and attenuator 36 for phase shifting and attenuating the input signal received on input terminal 31, and in which path 33b includes delay means 347 for delaying the input IF signal. The output signals from paths 33a and 33b are combined to provide an output IF signal on output terminal 39 which simulates multi-path fading.
Both of these prior art techniques for providing a multi-path simulation signal at IF radio frequencies are subject to significant drift and instability problems over time and temperature changes, making frequent calibration a necessary requirement. More specifically, the circuits which control the phase shifters and attenuators are very sensitive to temperature. For example, for a desired 40 dB notch depth to be accurate within a desired 1 dB, the attenuation must be very close as shown in Table 1, where B and b are defined in equation (1).
TABLE 1 ______________________________________ Error for 1 dB Notch Depth Depth B b in dB 1 dB error .1 dB error ______________________________________ -10 dB -3.302 .46 dB .046 dB -20 -.915 .11 .011 -30 -.279 .033 .003 -40 -.087 .010 .001 -50 -.028 -60 -.009 -70 -.003 -80 -.001 ______________________________________
In other words, in order to hold a 40 dB notch to within 1 dB, the circuit must be sufficiently stable to cancel the two signals from paths 33a and 33b (FIG. 3) to 60 dB (i.e. a 40 dB notch held to 1 dB error corresponds to b=0.10 db, which is approximately equal to b=0.009 dB for a 60 dB notch depth. This serves as an example of the magnitude of the stability required to provide accurage notch depth. Similar problems exist in holding the center frequency of the notch, as determined by the delay and phase shift to the desired frequency, as shown in Table 2.
TABLE 2 ______________________________________ Error for 100 KHz Notch Frequency Error Frequency total-phase 0.1 MHz error ______________________________________ 40 MHz 89.3 degrees .23 degrees 50 66.6 .23 60 43.9 .23 70 21.0 .23 80 -1.4 .23 90 -24 .23 100 -47 .23 ______________________________________
Prior art such as described by Richman, cited above, uses a similar circuit where the phase shifter and attenuator are perhaps combined. Richman's circuit injects a test signal and the notch center frequency and depth are adjusted to null out the injected test signal as measured with a detector at the output. An appropriate amount of signal is then added back from the desired channel to bring the level at the notch bottom up to the required level, using a comparison loop with a precision attenuator.
Feedback loops hold the notch frequency and depth to the required precision. Two problems with Richman's circuit are: the obvious addition of complexity and cost, and the fact that the the circuit has a test signal that is in the band of the desired input signals. The feedback loop will thus react in an undesirable way with the input signals so as to act other than as a passive transfer function. In addition, the proper operation of the system requires the presence of a particular type of signal on the input so that other signals will not work. One particular type of signal that will not work is that provided by a network analyzer, so that the performance of the circuit is not easily tested, thereby adding to the cost of production and testing the multi-path simulator.